In recent years, pipes and films made of plastics, injection-molded products and blow-molded products have been actively used in various industrial fields. Particularly polyethylene type resins have been used widely from such reasons as the low cost and light weight, excellence in molding processability, chemical resistance and recyclability.
In order to improve molding processability, since the molding is carried out in a molten state of polyethylene, emphasis is put on improvement in melt flow characteristics such as melt fluidity (easy-extrudability), melt extensibility and melt tension. For example, {circle around (1)} a method of broadening the molecular weight distribution by a multistage polymerization method employing a conventional Ziegler catalyst or further incorporating a specific molecular weight component (JP-A-2-53811, JP-A-2-132109, JP-A-10-182742), {circle around (2)} a method of using a traditional Cr type catalyst to produce a polyethylene having a long chain branching or adding a radical generator and a crosslinking aid to a resin to introduce a long chain branching (JP-B-2-52654), {circle around (3)} a method of using a polyethylene having a high melt extension stress to improve uniform extensibility (JP-A-10-7726) and the like have been proposed. However, there are many problems in the method {circle around (1)} such that the molded product tends to be more sticky or the impact strength tends to decrease by increase of a low molecular weight component, or gel is likely to form by increase of a high molecular weight body, there is such a problem in the method {circle around (2)} that the impact strength tends to decrease, and there is such a problem in the method {circle around (3)} that ESCR is not sufficient yet, although molding processability and impact strength are improved.
Further, in order to improve mechanical properties, {circle around (1)} a method of carrying out such a control that a low molecular weight component alone is decreased while maintaining a broad molecular weight distribution by improvement of the multistage polymerization or the catalyst with respect to a conventional Ziegler catalyst product to improve impact strength while maintaining molding processability (JP-A-7-90021), {circle around (2)} a method of introducing an α-olefin into a specific molecular weight component by multistage polymerization to improve ESCR (JP-A-10-17619), {circle around (3)} a method of using a metallocene catalyst which was developed in recent years to improve mechanical properties (JP-A-8-59741, JP-A-11-60633) and the like have been proposed. However, even by these methods, no ethylene polymer having both excellent mechanical properties and molding processability at a high level has been obtained.
It is an object of the present invention to provide a novel ethylene polymer excellent in molding processability represented by uniform extensibility, drawdown resistance and swell, and mechanical properties represented by rigidity, impact resistance and ESCR.